The medium of this invention is an improved medium designed for use with the optical detection system disclosed in U.S. applications Ser. Nos. 255,533 filed May 22, 1972 now abandoned and 461,249 filed Apr. 16, 1974 now U.S. Pat. No. 3,963,355 and in the improved devices disclosed and claimed in applications filed on even date herewith by Charles, Jones, Staples and Wiegner entitled AUTOMATED MICROBIAL ANALYZER and MACHINE AND PROCESS FOR READING CARDS CONTAINING MEDICAL SPECIMENS. These applications describe mechanism and apparatus suitable for analyzing specimens for specific microorganisms using a plastic tray or card which contains a series of dried culture media contained in separate but connected wells, each of the media being specific to a single organism. When the sample is inserted into the card, mixed with the media in the wells, and incubated in the machine; the organism (or organisms) present in the specimen interacts with the culture medium specific to that organism and produces a change in the medium which is read by the machine to indicate the presence of that organism. The change in the medium involves a change in the light transmitting properties of the medium, i.e., a color change or change in turbidity. The change may be caused by metabolic activity of the organism, which, for example, may cause production of acid and a change in pH which causes a color change in a pH sensitive indicator in the medium. The change in the light transmitting properties of the medium also could be caused by a precipitate forming in the medium due to metabolic activity of the organism or it could be caused by growth of the organism.
The specific media designed for use in the aforesaid cards are all designed to favor growth of one microorganism and to inhibit growth of other organisms, are capable of being freeze dried, and can function in the low 0.sub.2 environment of the wells of the card described in detail in said copending applications "AUTOMATED MICROBIAL ANALYZER" and "MACHINE AND PROCESS FOR READING CARDS CONTAINING MEDICAL SPECIMENS".
The medium of this invention is designed to favor growth of the gram negative bacterium Escherichia coli (E. coli). It contains sugar sources which are fermented selectively by E. coli to produce acids and further contains a pH sensitive indicator, reduced Aniline Blue, which turns from clear to a blue color in response to the change in pH produced by such acid production. The medium also contains a combination of inhibitors of other gram negative organisms, principally Klebsiella. The combination of inhibitors includes coumaric acid, 3,4-dihydroxy-benzoic acid, and saponin. This combination inhibits Klebsiella and allows E. coli to grow. The medium also contains surfactants as inhibitors for gram positive organisms.